blaze: Allocation of Resources Without Limits or Throttling
CVE-2026-73493 is a high-severity allocation of resources without limits or throttling vulnerability in blaze from Http4s, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected blaze versions: < 0.23.18; >= 1.0.0-M1, < 1.0.0-M42. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.63% chance of exploitation within 30 days, higher than 51% of scored CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of blaze you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CNA (github.com).
Official description
Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP services. Prior to 0.23.18 and 1.0.0-M42, http4s-blaze-server aggregates fragments of an incoming WebSocket message with no limit on total size or fragment count. A client that completes a WebSocket handshake can send an unterminated fragmented message and drive unbounded heap growth in the server JVM, resulting in denial of service through OutOfMemoryError. Any http4s application serving WebSocket routes over BlazeServerBuilder is affected, no non-default configuration is required, and maxWebSocketBufferSize does not bound the aggregate because it bounds only individual frames. A single connection sending continuation frames that never set FIN forces the server to buffer every fragment until the heap is exhausted, terminating the JVM with OutOfMemoryError on the blaze selector thread. Small fragments amplify the cost through per-frame object overhead, so a modest volume of wire bytes is sufficient. This issue is fixed in versions 0.23.18 and 1.0.0-M42.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Allocation of Resources Without Limits or Throttling
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
How developers prevent this class of flaw
- Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
- Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
- For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
More Allocation of Resources Without Limits or Throttling vulnerabilities (CWE-770)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Other vulnerabilities in blaze
- 7.4 High severity
- 7.4 High severity
Recent Allocation of Resources Without Limits or Throttling vulnerabilities
- 8.7 High severity
- 7.5 High severity
- 9.9 Critical severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
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